The present invention relates to the field of optical spectroscopy.
Spectroscopic techniques are widely used for determination of the composition of a substance. By spectrally analyzing an optical signal, i.e. a spectroscopic optical signal, the concentration of a particular compound of the substance can be precisely determined. The concentration of a particular substance is typically given by an amplitude of a principal component of an optical signal.
U.S. Pat. No. 6,198,531 B1 discloses an embodiment of an optical analysis system for determining an amplitude of a principal component of an optical signal. The known optical analysis system is part of a spectroscopic analysis system suited for, e.g., analyzing which compounds are comprised at which concentrations in a sample. It is well known that light interacting with the sample carries away information about the compounds and their concentrations. The underlying physical processes are exploited in optical spectroscopic techniques in which light of a light source such as, e.g., a laser, a lamp or light emitting diode is directed to the sample for generating an optical signal which carries this information.
For example, light may be absorbed by the sample. Alternatively or in addition, light of a known wavelength may interact with the sample and thereby generate light at a different wavelength due to, e.g., a Raman process. The transmitted and/or generated light then constitutes the optical signal which may also be referred to as the spectrum. The relative intensity of the optical signal as function of the wavelength is then indicative for the compounds comprised in the sample and their concentrations.
To identify the compounds comprised in the sample and to determine their concentrations the optical signal has to be analyzed. In the known optical analysis system the optical signal is analyzed by dedicated hardware comprising an optical filter. This optical filter has a transmission which depends on the wavelength, i.e. it is designed to weight the optical signal by a spectral weighting function which is given by the wavelength dependent transmission. The spectral weighting function is chosen such that the total intensity of the weighted optical signal, i.e. of the light transmitted by the filter, is directly proportional to the concentration of a particular compound. Such an optical filter is also denoted as multivariate optical element (MOE). This intensity can then be conveniently detected by a detector such as, e.g., a photo diode. For every compound a dedicated optical filter with a characteristic spectral weighting function is used. The optical filter may be, e.g., an interference filter having a transmission constituting the desired weighting function.
For a successful implementation of this analysis scheme it is essential to know the spectral weighting functions. They may be obtained, e.g., by performing a principal component analysis of a set comprising N spectra of N pure compounds of known concentration where N is an integer. Each spectrum comprises the intensity of the corresponding optical signal at M different wavelengths where M is an integer as well. Typically, M is much larger than N. Each spectrum containing M intensities at corresponding M wavelengths constitutes an M dimensional vector whose M components are these intensities. These vectors are subjected to a linear-algebraic process known as singular value decomposition (SVD) which is at the heart of principal component analysis and which is well understood in this art.
As a result of the SVD a set of N eigenvectors zn with n being a positive integer smaller than N+1 is obtained. The eigenvectors zn are linear combinations of the original N spectra and often referred to as principal components or principal component vectors. Typically, the principal components are mutually orthogonal and determined as normalized vectors with |zn|=1. Using the principal components zn, the optical signal of a sample comprising the compounds of unknown concentration may be described by the combination of the normalized principal components multiplied by the appropriate scalar multipliers:x1z1+x2z2+ . . . +xnZn,
The scalar multipliers xn with n being a positive integer smaller than N+1 may be considered the amplitudes of the principal components zn in a given optical signal. Each multiplier xn can be determined by treating the optical signal as a vector in the M dimensional wavelength space and calculating the direct product of this vector with a principal component vector zn.
The result yields the amplitude xn of the optical signal in the direction of the normalized eigenvector zn. The amplitudes xn correspond to the concentrations of the N compounds.
In the known optical analysis system the calculation of the direct product between the vector representing the optical signal and the eigenvector representing the principal component is implemented in the hardware of the optical analysis system by means of the optical filter. The optical filter has a transmittance such that it weights the optical signal according to the components of the eigenvector representing the principal component, i.e. the principal component vector constitutes the spectral weighting function. The filtered optical signal can be detected by a detector which generates a signal with an amplitude proportional to the amplitude of the principal component and thus to the concentration of the corresponding compound.
In a physical sense, each principal component is a constructed “spectrum” with a shape in a wavelength range within the optical signal. In contrast to a real spectrum, a principal component may comprise a positive part in a first spectral range and a negative part in a second spectral range. In this case the vector representing this principal component has positive components for the wavelengths corresponding to the first spectral range and negative components for the wavelengths corresponding to the second spectral range.
In an embodiment the known optical analysis system is designed to perform the calculation of the direct product between the vector representing the optical signal and the eigenvector representing the principal component in the hardware in cases where the principal component comprises a positive part and a negative part. To this end, a part of the optical signal is directed to a first filter which weights the optical signal by a first spectral weighting function corresponding to the positive part of the principal component, and a further part of the optical signal is directed to a second filter which weights the optical signal by a second spectral weighting function corresponding to the negative part of the principal component. The light transmitted by the first filter and by the second filter are detected by a first detector and a second detector, respectively. The signal of the second detector is then subtracted from the signal of the first detector, resulting in a signal with an amplitude corresponding to the concentration.
In another embodiment the known optical analysis system is able to determine the concentrations of a first compound and of a second compound by measuring the amplitudes of a corresponding first principal component and of a second principal component. To this end, a part of the optical signal is directed to a first filter which weights the optical signal by a first spectral weighting function corresponding to the first principal component, and a further part of the optical signal is directed a second filter which weights the optical signal by a second spectral weighting function corresponding to the second principal component. The light transmitted by the first filter and by the second filter are detected by a first detector and a second detector, respectively. The signal of the first detector and of the second detector correspond to the amplitudes of the first principal component and of the second principal component, respectively.
It is a disadvantage of the known optical analysis system that the signal to noise ratio is relatively low.
The present invention therefore aims to provide an optical analysis system of the kind described above, which is able to provide a signal with a relatively high signal to noise ratio.
The present invention provides an optical analysis system for determining an amplitude of a principal component of an optical signal. The optical analysis system comprises a first multivariate optical element, a second multivariate optical element, a first and a second detector. The first multivariate optical element is adapted for wavelength selective separation of the optical signal into a first part and a second part. The second multivariate optical element is adapted for wavelength selective weighting of the optical signal on the basis of a spectral weighting function. In particular, weighting of the optical signal refers to weighting of the first part and the second part of the optical signal. The first detector is adapted for detecting the weighted first part of the optical signal and the second detector is adapted for detecting the weighted second part of the optical signal respectively.